Fractures are pre-existing planes of weaknesses in rock units. They play a critical role in determining, for example, the production potential of reservoirs that are developed through lateral/horizontal drilling and hydraulic fracturing (“fracking”). As a specific example, knowledge of fracture orientation and/or extent is key to production in formations such as the Mid-Continent Mississippian Limestone. Rock fractures contribute to porosity and, if they are open, have the potential to contribute substantially to the permeability of a formation. Obviously, permeability in producing formations is necessary and highly desirable as it provides a pathway for oil to be pushed or drawn to the borehole so that it can be extracted. Similarly, where there is knowledge or an estimate of where a reservoir is fractured internally is an important consideration in the process of selecting a drill site for exploration or production purposes.
Estimation of spatial fracture density using geophysical methods, such as seismic, is highly desirable but extremely challenging theoretically and in practice. Attributes of seismic data such as amplitudes, phase and frequency originate from a complex interaction between the propagating seismic wavefield and the rocks and fluids the waves encounters along their path. Individual rock properties such as fracture density, mineralogy, porosity, permeability and fluid saturation type have varying degree of influence on individual seismic attributes. Thus, a single seismic attribute cannot be uniquely related to any particular rock property.
Of course, seismic data interrogates the subsurface of the earth without drilling and, to the extent such could be used to estimate fracture density, such knowledge could be influential in the decision as to whether and where to drill. Additionally, knowledge of the distribution and orientation of exiting fractures would be useful in planning hydraulic fracking operations, offset wells in producing formations, injection wells, etc.
Thus, what is needed is a methodology for estimating subsurface fracture density using seismic that is not subject to the disadvantages of prior art approaches.
Before proceeding to a description of the present invention, however, it should be noted and remembered that the description of the invention which follows, together with the accompanying drawings, should not be construed as limiting the invention to the examples (or embodiments) shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of this invention within the ambit of the appended claims.